I recently upgraded my Comcast service from 200 Mbps to 300 Mbps.  After the upgrade, I measured my download speed at 276 Mbps.  This corresponds exactly to the expected speed for my Zoom 5345 cable modem as shown on the Comcast approved equipment list.

To get the last bit of speed, I installed a Motorola MB7621 cable modem.  In order to avoid problems with Comcast support, I connected it directly to my linux desktop.  After it was activated by Comcast, I measured my download speed at 370 Mbps.  However, when I connected my normal way through the R600VPN, the speed dropped back to ~200 Mbps.  Retrying the direct connection confirms that the R600VPN is the bottleneck.

One thing to note is that while running the speed test (http://www.dslreports.com/speedtest) it initially spikes well over 300 Mbps for a few seconds before settling to ~200 Mbps.  Other speed test websites give the same measurement.

Any ideas what's going on?  Are there configuration settings to check to ensure max performance?  Are my hardware/firmware versions known to cause problems?
 

Thanks in advance.

Kev